The invention relates to current limiting fuses.
A fuse is a current interrupting device which protects an electrical circuit in which it is installed by creating an open circuit condition in response to excessive current (i.e., a current which exceeds the maximum allowable continuous current for the fuse). The current is interrupted when the element or elements which carry the current are melted by heat generated by the current. The fuse may contain materials, such as silica sand, which serve to reduce the time required for the fuse to interrupt the current.
A fuse""s maximum allowable continuous current is a function of the size and number of current carrying elements in the fuse. One way to increase the maximum continuous current rating of a fuse is to place elements in parallel along the length of the fuse. A winding support may be used to hold the elements in place with proper element spacing. Typically, a minimum element length (e.g., 1.8 inches) per kilovolt of voltage is needed for proper fuse operation. Elements of sufficient length often are wound in a spiral manner to maintain a reasonable fuse length, since such spiral winding permits the element length to be longer than the fuse length.
An element winding support often includes two or more intersecting plates that intersect at their midsections. Typically, metallic plates are attached to the ends of the support. The fuse elements are attached to the plates in a way that provides essentially zero impedance. The plates in turn are secured to caps that fit over ends of a tube that holds and supports the internal structure of the fuse. The tube is filled with silica sand, which is compacted. Finally, the fill point of the fuse, which is typically a hole in one of the caps, is then sealed.
A fuse includes a housing having ends to which electrically-conductive caps are attached. At least two winding support assemblies are positioned in the housing and extend between the caps. The winding supports are spaced from one another. Each winding support is wound by at least one electrically-conductive element that is electrically connected to the caps.
Embodiments may include one or more of the following features. For example, each winding support may include one or more additional electrically-conductive elements, each of which is wound around the winding support and electrically connected to the caps, so that, for example, each winding support carries two or more elements. A winding support may include two or more plates that act in concert to form multiple winding support surfaces. The fuse also may include additional winding supports and associated elements.
Each winding support may include a minor axis component and a major axis component that fit together to form the winding support. A width of the major axis component is greater than a width of the minor axis component to render the winding support asymmetrical. Each component may include at least one slotted opening that allows the components to fit together to form the winding support. The components also may include exterior nibs to retain the elements in place.
Each winding support may be made of, for example, mica, plastic, or a ceramic material. As an alternative to being formed from a pair of slotted plates, the winding support may be a single piece of plastic or ceramic material formed in the desired shape. For example, a plastic support may include three or more support arms. A winding support also may be made from non-interlocking plates that are clipped to end support plates.
Multiple winding supports may be formed using a single winding support structure. For example, two four-armed winding supports may be defined by a slotted central plate and a pair of cross plates. The slotted central plate if defines two arms of each of the winding supports, while each cross plate defines the other two arms of one of the winding supports.
Typically, the winding supports are non-coaxial. The winding supports may extend in directions generally parallel to a longitudinal central axis defined by the housing. The winding supports also may be offset from the central axis. The housing may be cylindrical in shape, and may contain silica sand. The housing may contain a pair of plates, with each cap physically connected to a plate and each plate physically connected to the winding supports.
By using multiple winding supports, the fuse is able to carry two to three times more current than may be carried by a traditional fuse employing a housing of the same size.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.